Simplify the following expression: $n = \dfrac{1}{4p} - \dfrac{4}{9p}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $4p$ and $9p$ $\lcm(4p, 9p) = 36p$ $ n = \dfrac{9}{9} \cdot \dfrac{1}{4p} - \dfrac{4}{4} \cdot \dfrac{4}{9p} $ $n = \dfrac{9}{36p} - \dfrac{16}{36p}$ $n = \dfrac{9 -16}{36p}$ $n = \dfrac{-7}{36p}$